--> --> ABSTRACT: Crude Oil and Source Rock Correlation, Northwest Colorado, by G. E. Michael, D. E. Anders, W. Dickinson; #91002 (1990).

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ABSTRACT: Crude Oil and Source Rock Correlation, Northwest Colorado

G. E. Michael, D. E. Anders, W. Dickinson

The northwest Colorado area of the Piceance and Sand Wash basins, Axial uplift, and Douglas Creek arch is characterized by thermally mature to overmature rocks, and contains Colorado's largest oil field, Rangely (750 MMBO), and several smaller oil and gas fields. Oil-oil and oil-source rock correlation in northwest Colorado is difficult because of high maturity of the oils and probable source rocks, wherein many of the standard correlation parameters are obscured. More than 40 source-rock samples of Pennsylvanian and Permian age (Belden and Phosphoria formations, respectively) and Cretaceous age (Mowry, Niobrara, and Mancos formations) were analyzed. Forty-nine oils from Cretaceous, Jurassic, Triassic, and Pennsylvanian age reservoirs and ten different fields were analyze .

Sterane and terpane biomarkers, saturate and aromatic hydrocarbon distributions, and 13C isotope analyses of saturate and aromatic fractions were used to make oil-oil and oil-source rock correlations. API gravity for the oils varied from 30.1° to 57.8° with most values between 35° and 45°. Pennsylvanian Minturn Formation reservoir oils are most mature (API 43.7° to 57.8°) and show no preservation.

Based on geochemical data, three families of oils exist in northwest Colorado. Family I oils, of marine organic matter origin, are distinguished by their unique tri- and tetracyclic terpane distribution and relatively high amount of aromatic sulfur compounds. Family I oils are found primarily in the Pennsylvanian Weber Formation and in some Mesozoic reservoirs at Rangely field. but family I oils are also found in some fields east of Rangely in Weber and Mesozoic reservoirs. The Phosphoria Formation is the probable source for family I oils.

Family II oils are of terrestrial/marine organic matter source and are found primarily in Cretaceous reservoirs. Adjacent Cretaceous shales are considered to be the source for family II oils. Shales of the Belden Formation, deposited under deltaic conditions, could be another source for family II oils, but the shales are difficult to assess because of high thermal maturity.

Family III oils are restricted in the White River Dome field (Piceance basin) and Powder Wash field (Sand Wash basin) and are in Upper Cretaceous Mesaverde Group or Fort Union Formation reservoirs, respectively. A very low sterane:hopane ratio and a high pristane:phytane ratio indicate family 111 oils were derived from a mostly terrestrial source. Cretaceous shales may be the primary source for family III oils, but significant input could have come from Mesaverde Group coals.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990